German Patent Application No. 199 43 887 has already described a rain sensor, in which the light of an infrared transmitter is radiated from the inside of the motor vehicle out in the direction of the outside through the laminate safety glass. At the outside of the glass—assuming a clean glass—the light is totally reflected and returned inwards. Disposed in the intermediate layer of the laminate safety glass is a retro-reflector which returns the light offset by a small angle, so that it may be coupled out in the region of the transmitter.